The present invention relates generally to the field of cellulosic pulp processing, and more specifically to the processing of cellulosic pulp to prepare nanocellulose fibers, also known in the literature as microfibrillated fibers, microfibrils and nanofibrils. Despite this variability in the literature, the present invention is applicable to microfibrillated fibers, microfibrils and nanofibrils, independent of the actual physical dimensions.
Nanofibrillated celluloses have been shown to be useful as reinforcing materials in wood and polymeric composites, as barrier coatings for paper, paperboard and other substrates, and as a paper making additive to control porosity and bond dependent properties.
Conventionally, chemical pulps produced using Kraft, soda or sulfite cooking processes have been bleached with chlorine-containing bleaching agents. Although chlorine is a very effective bleaching agent, the effluents from chlorine bleaching processes contain large amounts of chlorides produced as the by-product of these processes. These chlorides readily corrode processing equipment, thus requiring the use of costly materials in the construction of bleach plants. In addition, there are concerns about the potential environmental effects of chlorinated organics in bleach plant effluents. Other known pretreatment processes include oxygen-based compounds, such as ozone, peroxide and oxygen, for the purpose of delignifying, i.e. bleaching pulp.
The bleaching and other pretreatment of pulps however is distinct from and, by itself, does not result in release of nanocellulose fibers. A further mechanical refining or homogenization is typically required, and refining processes are generally divided into high and low consistency, which refers to the solids content of the pulp slurry being considered. Low consistency refining generally consists of 2-6% by weight solids. Mechanical refining requires a great deal of energy to mechanically and physically break the cellulose fibers into smaller fragments. Required energy is a complex mix of many variables related to the refiner itself, the pulp mixture to be refined, and the configuration of the refiner blades, or plates. According to one popular theory, specific edge loading, (SEL) is a useful measure of the “intensity” of refining. It contemplates both the number of impacts and the intensity of the impacts that a fiber “sees” during one revolution of the refiner plates. The number of impacts (as a rate) is related to the blade configuration and is given by the total cutting edge length per rotation (CEL) and rotational speed. The intensity of such impacts is related to the energy transferred to the fiber, or “net” power consumption, and is given by the total power applied minus the no-load power, or (p−p0). Thus, the SEL may be defined as the effective energy expended per bar crossing per unit bar length. The mathematical definition is shown in the equation below, where Ω is the rotational speed of the refiner and other terms are as defined above.SEL=(p−p0)/Ω*CEL.SEL units are given in Watt-seconds/meter (Ws/m) or the equivalent Joules/meter (J/m).
Frequently multiple stages of homogenization or refining, or both, are required to achieve a nano-sized cellulose fibril. For example, U.S. Pat. No. 7,381,294 to Suzuki et al. describes multiple-step refining processes requiring 10 or more, and as many as 30-90 refining passes. The refining passes or stages may use the same or different conditions. The process described by Suzuki et al generally produces fibers having a length of 0.2 mm or less, by many refiner passes, resulting in very high specific energy consumption, for both pumping and refining operations. Suzuki's teaching does not take into account the intensity of the impacts and does not calculate the SEL.
A second example is provided by US 2014/0057105 to Pande et al. in which fibers are refined in one or more stages to increase hydrodynamic surface area without a substantial reduction in fiber length.
It would be advantageous if there could be developed improved processes for cellulosic pulp processing, particularly a process that reduced the energy required to produce nanofibrils. Longer fibers are also preferred for some applications.